1. Field of the Invention
The present invention relates to a method for preparing 4,4'-biphenol. More particularly, the present invention relates to an economical method for preparing 4,4'-biphenol with high purity.
4,4'-Biphenol of the desired product to be prepared by the method of the present invention has recently been attracting a great deal of attention as a starting material for heat-resistant engineering plastics and the like. With the method of the present invention 4,4'-biphenol with enough purity for use as a raw material for these plastics can be prepared economically.
2. Description of the Background Art
4,4'-biphenol (referred to as BPL hereinafter) has been prepared by the following methods: alkali fusion of diphenyl disulfonic acid; hydrolysis of dibromodiphenyl; oxidative dimerization of 2,6-ditert-butyl phenol (referred to as 2,6-DTBP hereinafter) to 3,3',5,5'-tetra-tert-butyl biphenol (referred to as TBBPL hereinafter) and subsequent debutylation thereof; and the like. In the former two methods severe reaction conditions are required and separation of large amounts of, inorganic salts used in these reactions from the products is difficult, and therefore the oxidative dimerization of 2,6-DTBP is most preferably used to prepare BPL on an industrial level.
However, the process to produce BPL through the 2,6-DTBP route has not yet been fully economical or effective, particularly because the debutylation reaction has been proceeded in a batch process. The major problem of the debutylation reaction is the large endothermic reaction which requires large amounts of heat input to proceed with the reaction.
Consequently, the batch reactor requires a large of heat supply capacity to get high yields and quality of BPL. 0n the other hand, continuous processes using one reactor, which have never been known to give high yields with good quality, require very severe conditions, such as high temperatures, long detention time, and high concentrations of catalysts, which cause low selectivity and low quality of BPL.